1. Field of the Invention
This invention is directed to seismic streamers; to seismic streamer depth control devices; to methods of their use; and to methods of their storage and deployment.
2. Description of Related Art
The prior art discloses a wide variety of depth control devices for seismic streamers, some of which are referred to as "birds." Typically, a bird is suspended from a streamer from a slip ring attached to the streamer and is releasably connected thereby to the streamer as it is deployed and removed from the streamer as it is recovered. To control streamer depth, certain prior art systems use one bird for every thousand feet of streamer or for every three "sections."
Currents, wind, and wave action deflect the streamer cables from their intended paths, and streamer cable drift is a continuing problem for marine seismic surveys. See, for example, U.S. Pat. No. 5,532,975. By controlling the position and shape of the streamer cables entanglement of the streamer cables is avoided and collisions with offshore hazards such as marine drilling rigs and production platforms are prevented. Since known 3-D seismic binning processes acquire subsurface seismic coverage by combining seismic data from seismic streamers at different locations, it is desirable to have the ability to control the position and shape of the streamer cables during marine 3-D seismic surveys. The need for this ability is taught by Franklyn K. Levin in "Short Note: The effect of binning on data from a feathered streamer,"Geophysics, Vol. 49. No. 8, pp. 1386-1387.
A variety of streamer positioning devices are well known in the art. Apparatus, such as those disclosed in U.S. Pat. Nos. 5,532,975, 4,729,333, and 4,463,701 are attached to streamer cables to maintain them at a lateral offset to the pathway of a towing vessel. U.S. Pat. No. 4,890,568 describes steerable tail buoys for controlling the position of the tail end of towed seismic streamer cables. Streamer positioning devices that are attached externally to the streamer cables to control lateral positioning by using camber-adjustable hydrofoils or angled wings are disclosed in U.S. Pat. Nos. 4,033,278 and 5,443,027. U.S. Pat. No. 3,931,608 discloses a bird for controlling the vertical positioning of streamer cables with diving planes and a preset depth control apparatus.
The prior art discloses a variety of location sensing devices and methods for determining the positions of seismic sources and seismic streamer cables. A Global Positioning System, described in U.S. Pat. No. 4,809,005 and a network of acoustic elements described in U.S. Pat. No. 4,912,682 may be deployed on a vessel, streamer cables, and tail buoy and may then be used to determine the real-time position of seismic sources and seismic streamer cables by computing a network solution with a Kalman filter, e.g. as disclosed by U.S. Pat. No. 5,353,223.
In known prior art methods of marine seismic surveying, a human operator monitors the survey's operational conditions, such as the extent of subsurface seismic coverage, the adequacy of separations between streamer cables, and the proximity of streamer cables to obstructive hazards. When these conditions indicate the need to reposition the streamer cables, the operator may manually issue commands to various individual streamer positioning devices in order to adjust the position and shape of the streamer cable, or order a vessel's helmsman to redirect the vessel, or suspend data acquisition.
In certain prior art systems, a bird is about four feet long and is powered by multiple D-size batteries. Rechargeable batteries have been employed in some birds to extend the life of the batteries. Electric power in some devices is inefficiently transmitted to the bird via electromagnetic induction coils beneath the skin of the streamer and a coil in the bird. These birds must be removed from a streamer as it is retrieved and secured to the streamer as it is deployed. Communication between the ship-board streamer controller and the birds is accomplished via signals communicated at relative low data rates across the skin of the streamer by indirect means of one or more electromagnetic coils beneath the skin of the streamer and another coil within the bird structure. Elaborate mechanical schemes have been devised to assure communications as the streamer cable rotates relative to the bird.
There has long been a need for an easily deployable and retrievable seismic streamer whose depth can be controlled. There has long been a need for such a streamer with effective and efficient depth control devices. There has long been a need for a seismic streamer depth control device whose power is effectively and efficiently obtained from a streamer power distribution system. There has long been a need for a simple way to communicate directly with a bird's control and sensor electronics. There has long been a need for a bird device which is also capable of producing vertical, non-vertical, and horizontal forces which facilitate vertical and horizontal movement of the streamer cable. There has long been a need, recognized by the present inventors, for relatively compact bird devices that do not interfere with streamer deployment and retrieval operations and which, in certain aspects, remain on a streamer in storage.